6.8x10^5? Scientific notation
A solid will start to melt when it reaches its melting point. For example the melting point of Helium is -272° Celsius and the melting point of Carbon is 3500° Celsius . Helium has the lowest melting point and carbon has the highest melting point in the periodic table.
Max Planck concluded that energy is not continuous and is carried in discontinuous units which he named quanta.